Parametric microwave noise generator



United States Patent PARAMETRIC MICROWAVE NOISE GENERATOR Sungik Francis Paik, Sudbury, Mass., assignor to the United States of America as represented by the Administrator of the National Aeronautics and Space Administration Filed Mar. 11, 1968, Ser. No. 711,971 Int. Cl. H03b 29/00 US. Cl. 331-78 7 Claims ABSTRACT OF THE DISCLOSURE ORIGIN OF THE INVENTION The invention described herein was made by an employee of the United States Government and may be manufactured and used by or for the Government for governmental purposes without the payment of any royalties thereon or therefor.

BACKGROUND OF THE INVENTION This invention relates generally to a generator for producing radio frequency noise in the microwave frequency range and, more particularly, to such a generator which produces a noise output with a controllable amplitude and frequency spectrum.

Several applications exist wherein one desires a noise signal having given characteristics. For example, during test of electromagnetic wave receivers it is frequently desirable to determine sensitivity of the receiver in the presence of noise. Noise signals also are useful in various types of electronic countermeasure systems.

Previous microwave frequency noise generators have included, for example, systems utilizing electron beam switching tubes, one or more traveling wave tube amplifiers which amplify the noise in the microwave frequency spectrum produced by a gas discharge tube, traveling wave tubes operating without an input signal so as to amplify only their own noise, etc. These previous systems have exhibited either individually or collectively certain certain undesirable characteristics such as nonadjustable output noise signal amplitudes and frequency spectrum, insuflicient noise output power densities or extensive signal amplification requirements and failure to provide a completely random thermally generated noise signal.

The object of this invention, therefore, is to provide a relatively simple radio frequency noise generator capable of producing a microwave noise signal of adjustable amplitude and frequency spectrum.

A primary feature of this invention is the provision of a radio frequency noise generator including a slowwave structure, a gas discharge plasma generator and a large-signal pump wave generator for producing pump waves at a microwave frequency much higher than the resonant frequency of the plasma provided by the plasma generator. Pump waves propagated on the slow-wave structure couple with the electrostatic modes of oscillations in the plasma medium through a nonlinear coupling mechanism. As a result of the nonlinear coupling between the externally applied electromagnetic pump waves ice and the spontaneously occurring oscillations in the plasma, idler waves representing difference frequency components are generated parametrically in the microwave frequency range. Furthermore, if the phase velocities of the three waves are properly adjusted, the electrostatic and the idler waves are amplified according to the principal of the traveling-wave parametric amplifier. Thus, the entire spectrum of the normally nonradiative electrostatic plasma oscillations are converted into radiative noise represented by the parametrically amplified idler wave output of the slow-wave structure.

Another feature of this invention is the provision of a radio frequency noise generator of the above featured type wherein the slow-wave structure is a helix and the pump signal generator is a voltage tunable magnetron. The helix slow-wave structure is particularly well suited for this application as is the voltage tunable magnetron which permits selective adjustment of both the amplitude and spectrum center of the noise output represented by the idler waves.

Another feature of this invention is the provision of a radio frequency noise generator of the above featured type including a low pass filter circuit connected to the output of the slow-wave helix. The filter circuit removes the pump signal from the output leaving a random noise output represented only by the idler signal.

These and other features and objects of the invention will become more apparent upon a perusal of the following specification taken in conjunction with the accompanying drawings wherein:

FIGURE 1 is a schematic diagram illustrating a preferred radio frequency noise generator embodiment; and,

FIGURE 2 is a graph illustrating the phase velocity relationships and the spectral relationships between the waves generated in the generator of FIGURE 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIGURE 1 there is shown the noise generator 11 including the vacuum tight envelope 12 containing cesium atoms at a pressure of, for example, between 1O 10 mm. Hg. Disposed within the envelope 12 are the spaced apart gas discharge initiating plates 13 formed of a suitable material such as tungsten or tantalum. Heating of the plates 13 is provided by the adjacent heater elements 14.

Straddled by the plates 13 within the envelope 12 is the slow-wave helix structure 15. The input of the slowwave structure 15 is connected to the voltage tunable magnetron 16 and its output is connected to the series combination of the low pass filter network 17, the amplifier 18 and the antenna 19. After energization of the heater elements 14, cesium atoms contacting the hot plates 13 are ionized generating a quiescent plasma which immerses the slow-wave helix structure 15.

Operation of the high frequency noise generator 11 will be explained in connection with the graph of FIGURE 2 wherein frequency w is plotted on the vertical axis and wave number k is plotted on the horizontal axis. The solid line 21 approximates the dispersion characteristics (w-k relationship) of the slow-wave helix 15 and the shaded area 22 represents the dispersion characteristics of the electrostatic waves in the plasma generated by the plates 13. The plasma dispersion characteristic 22 is determined by the density and temperature of the plasma electrons and these parameters preferably are adjusted such that the plasma resonant frequency w is at least one order of magnitude lower than the frequency-band in which the noise output is desired. The magnetron 16 is adjusted to provide on the slow-wave structure 15 a large-signal pump wave at a microwave frequency (.0 much higher than the plasma frequency w Having phase velocities substantially equal to that of the signal pump wave, the plasmas electrostatic oscillations within the frequency band from to to will interact with the pump signal producing idler waves in the frequency spectrum between (i1 and 01 Because of the substantially straight line dispersion characteristic of the helix structure 15, these idler waves also have phase velocities substantially equal to that of the pump signal and are parametrically amplified thereby.

Thus, in a particular example indicated in FIGURE 1, the nonradiative electrostatic plasma oscillations in the frequency spectrum between 500 and 1000 megacycles are converted and amplified by a pump signal at a frequency of 3000 megacycles into radiative idler waves in a frequency spectrum between 2000 and 2500 megacycles. Obviously, the center of the idler wave bandwidth is directly dependent upon the frequency of the pump signal. Also, since the idler waves are amplified parametrically, the amplitude of the noise output represented thereby will depend on both the length of the slow-wave structure and the input pump power. Therefore, both amplitude and frequency spectrum of the noise output can be directly controlled by adjustment of the magnetron 16 to produce a pump signal of given input power and frequency. After elimination of the pump signal in the low pass filter 17, the noise output from the antenna 19 is limited to the desired frequency spectrum represented by the idler wave output.

Although the substantially straight line dispersion characteristics of the slow-wave helix structure is particularly desirable, it will be obvious that other slow-wave structures could be utilized within the scope of the invention provided that the conditions required for parametric amplification are satisfied. The conditions will be satisfied if the relationships w +t =w and k -l-k k are both satisfied where w to; and (0 represent frequencies and k k, and k represent wave numbers, respectively, of the electrostatic waves, the idler waves and the pump waves. In addition, gas discharge plasmas other than a cesium thermal discharge could be utilized as a medium in which the required electrostatic waves are generated.

Obviously, many modifications and variations of the present invention are possible in light of the above teachings.

What is claimed is:

1. A radio frequency noise generator comprising a vacuum sealed envelope, microwave slow-wave structure means disposed within said envelope, plasma generating means disposed within said envelope and adapted to generate a gas discharge plasma therein, and signal generator means for producing on said slow-wave structure electromagnetic pump waves which extract random electrostatic energy from the gas discharge plasma.

2. A radio frequency noise generator according to claim 1 wherein said random electrostatic energy is extracted as parametrically amplified idler waves produced by interaction between the pump waves and the electrostatic plasma oscillations.

3. A radio frequency noise generator according to claim 2 wherein said signal generator produces a pump wave frequency substantially higher than the resonant oscillation frequency of the gas discharge plasma so as to provide an idler wave output spectrum in the microwave frequency range.

4. A radio frequency noise generator according to claim 3 wherein said signal generator provides a selectively variable pump signal frequency so as to permit control of the idler wave output spectrum.

5. A radio frequency noise generator according to claim 4 wherein said signal generator comprises a voltage tunable magnetron.

6. A radio frequency noise generator according to claim 5 wherein said slow-wave structure means comprises a helix.

7. A radio frequency noise generator according to claim 6 including a low pass filter circuit connected to the output of said slow-wave structure and adapted to eliminate the pump signal component from the slowwave structure output.

References Cited UNITED STATES PATENTS 2,726,332 12/1955 Arditi et a] 33178 X 3,356,965 12/1967 Agdur et al 331-78 -ROY LAKE, Primary Examiner SIEGFRIED H. GRIMM, Assistant Examiner US. Cl. X.R. 33174, 77, 126 

